sched: clean up fixup_hmp_sched_stats()

The commit 392edf49 ("sched: avoid stale cumulative_runnable_avg
HMP statistics) introduced the callback function fixup_hmp_sched_stats()
so update_history() can avoid decrement and increment pair of HMP stat.
However the commit also made fixup function to do obscure p->ravg.demand
update which isn't the cleanest way.

Revise the function fixup_hmp_sched_stats() so the caller can update
p->ravg.demand directly.

Change-Id: Id54667d306495d2109c26362813f80f08a1385ad
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>